1. Field of the Invention
This invention relates generally to a system and method for maintaining the voltage of fuel cells in a fuel cell stack below a predetermined maximum voltage threshold and, more particularly, to a system and method for maintaining the voltage of fuel cells in a fuel cell stack below a predetermined maximum voltage threshold by determining a predicted minimum gross power feed-forward term using parameters determined from a polarization curve estimation and charging a battery and/or applying an auxiliary load to the stack to reduce the average or maximum voltage measurement of the fuel cells below the maximum voltage is necessary.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
A fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
It has been discovered that a typical fuel cell stack will have a voltage loss or degradation over the lifetime of the stack. It is believed that the fuel cell stack degradation is, among others, a result of voltage cycling of the fuel cells in the stack. The voltage cycling occurs when the platinum catalyst particles used to enhance the electro-chemical reaction transition between an oxidized state and a non-oxidized state, which causes dissolution of the particles. If the voltage of a fuel cell is less than about 0.9 volts, the platinum particles are not oxidized and remain a metal. When the voltage of the fuel cell goes above about 0.9 volts, the platinum crystals begin to oxidize. A low load on the stack may cause the voltage output of the fuel cell to go above 0.9 volts. The 0.9 volts corresponds to a current density of 0.2 A/cm2, depending on the power density of the MEA, where a current density above this value does not change the platinum oxidation state. The oxidation voltage threshold may be different for different stacks and different catalysts.
When the platinum particles transition between a metal state and an oxidized state, oxidized ions in the platinum are able to move from the surface of the MEA towards the membrane and probably into the membrane. When the particles convert back to the metal state, they are not in a position to assist in the electro-chemical reaction, reducing the active catalyst surface and resulting in the voltage degradation of the stack.
As discussed above, voltage cycling to near stack open circuit voltage (OCV) and sustained fuel cell stack operation at or near the stack OCV causes a reduction in platinum catalyst surface area and leads to corrosion of the catalyst support. By maintaining the average cell voltage below a certain predetermined threshold, such as 900 mV, it is possible to prevent voltage degradation in the stack and improve its durability.